Known in the art is an internal combustion engine which arranges, in an engine exhaust passage, an NOX storage catalyst which stores NOX which is contained in exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean and which releases the stored NOX when the air-fuel ratio of the inflowing exhaust gas becomes rich, which arranges a fuel addition valve in the engine exhaust passage upstream of the NOX storage catalyst, which arranges an air-fuel ratio sensor inside the engine exhaust passage downstream of the NOX storage catalyst, and which feeds fuel from the fuel addition valve to the inside of the engine exhaust passage to make the air-fuel ratio of the exhaust gas flowing into the NOX storage catalyst rich when NOX should be released from the NOX storage catalyst (for example, see Patent Literature 1).
In this regard, in this internal combustion engine, for example, if the fuel addition valve becomes clogged, the amount of fuel feed when feeding fuel from the fuel addition valve to release NOX becomes smaller compared with the amount of fuel feed when the fuel addition valve is not clogged. As a result, at this time, the air-fuel ratio of the exhaust gas which flows out from the NOX storage catalyst becomes the lean side compared with the air-fuel ratio when the fuel addition valve is not clogged. Therefore, in this internal combustion engine, the air-fuel ratio of the exhaust gas when fuel is fed from the fuel addition valve for release of NOX is detected by the air-fuel ratio sensor. When the air-fuel ratio of the exhaust gas which is detected by the air-fuel ratio sensor becomes the lean side compared with the air-fuel ratio when the fuel addition valve is not clogged, it is judged that there is an abnormality in the fuel addition valve.